The present invention relates to a process for the reductive dehalogenation of liquid and solid halogenated hydrocarbons as such or as contaminants in liquids and solids through chemical reaction with reducing metals in the presence of a hydrogen donating compound characterized in that the halogenated hydrocarbons or the liquids and solids, which contain the halogenated hydrocarbons, are transformed into a finely dispersed pulverulent solid formulation and said finely dispersed pulverulent solid formulation, which contains the halogenated hydrocarbons, is heated up in the presence of a finely dispersed reducing metal and a hydrogen donating compound to a temperature ranging from 80 to 400xc2x0 C.
Halogenated hydrocarbons in this context are aliphatic, aromatic and mixed aliphatic-aromatic hydrocarbons, which contain at least one halogen in a molecule, including those hydrocarbons, which, in addition to halogen, contain other functional groups, for instance chlorophenols.
Hydrogen donating compounds in this context are all compounds, which can provide hydrogen formally as protons or atomic hydrogen to saturate anions and radicals respectively, for instance alcohols, amines, aliphatic hydrocarbons.
Halogenated hydrocarbons are toxic. This can be attributed to the presence of halogen in the molecule. Accordingly, toxic halogenated hydrocarbons can be detoxified through removal of the halogen. Since one single halogen in a molecule can cause a disproportionate high toxicity, it is essential, therefore, to dehalogenate multiply halogenated hydrocarbons in such a way that all the halogen atoms will be removed. This is chemically possible only in case of a reductive dehalogenation.
Halogenated aromatics like polychlorinated biphenyls (PCB), polychlorinated phenols (PCP) and polychlorinated dibenzodioxins and dibenzofurans (PCDD/PCDF) are extremely toxic. PCDD/PCDF, which generally are subsumed under the term xe2x80x9cdioxinsxe2x80x9d are regarded as ultratoxic. Aliphatic halogenated compounds like hexochlorocyclohexane (HCH) or mixed aliphatic-aromatic compounds like trichloro-bis(chlorophenyl)-ethane (DDT) are also toxic even though they are not classified as ultratoxics in general. All these compounds quoted here are, along with other halogenated hydrocarbons, widely spread in the environment. Due to their bioavailability they own a high hazard potential which to abolish must be regarded as an important demand of environmental relevance.
There are a number of processes which deal with the reductive dehalogenation of halogenated hydrocarbons. According to EP 0 099 951 PCB is dehalogenated by means of finely dispersed molten sodium at 100 to 160xc2x0 C. According to U.S. Pat. No. 4,973,783 halogenated aromatics are reacted with alkali metal in the presence of hydrosiloxane. In U.S. Pat. No. 4,639,309 halogenated hydrocarbons are dehalogenated with sodium or potassium at 100xc2x0 C. with the aid of mechanical abrasion of the formed halogenides. According to U.S. Pat. No. 4,950,833 halogenated aromatics are reacted with alkali metal in the presence of ammonium salts at 40 to 60xc2x0 C. and according to the Canadian Application 2026506, in which a whole lot of further processes are quoted referring to the state of the art, halogenated aromatics are treated with finely dispersed sodium or calcium in the presence of methanol, ethanol or isopropanol at temperatures below 100xc2x0 C. A process of particular effectiveness seems to be described in EP 0 225 849, in which halogenated aliphatic or aromatic compounds are reacted with sodium in an inert solvent in the presence of a C1- to C5-alcohol at temperatures between 100 and 150xc2x0 C.
The quoted processes have some significant disadvantages. The most important drawback is that said processes are restricted to the use of alkali and alkali earth metals, further in that, as a rule, the reaction temperature must be above the melting point of sodium and that the reaction must be carried out in a homogenous liquid medium in an atmosphere of protecting gas. Said processes are, on no account, applicable to the solution of environmental problems in practice, for instance, to dehalogenate PCB in sludge, dioxins in wet soil, PCP in sand, HCH in a mix of waste material in landfills etc.
Therefore, it would be highly desirable to provide a process efficient to chemically and technically simply dehalogenate halogenated hydrocarbons of whatsoever origin as such or as contaminants in other materials without the restrictions and drawbacks as quoted above.
In accordance with the present invention there is now provided a process for the reductive dehalogenation of liquid and solid halogenated hydrocarbons as such or as contaminants in liquids and solids through chemical reaction with reducing metals in the presence of a hydrogen donating compound characterized in that the halogenated hydrocarbons or the liquids and solids, which contain the halogenated hydrocarbons are transformed into a finely dispersed pulverulent solid formulation and said finely dispersed pulverulent solid formulation, which contains the halogenated hydrocarbons, is heated up in the presence of a finely dispersed reducing metal and a hydrogen donating compound to a temperature ranging from 80 to 400xc2x0 C.
As reducing metals can be used preferably sodium, magnesium, aluminum and iron but also lithium, potassium, calcium and zinc and the alloys thereof in a finely dispersed form. This finely dispersed form of the metal can be added during the preparation of the finely dispersed, pulverulent solid formulation or during the heating of said solid formulation.
The invention is based on ideas which can be explained most simply by way of an example from practice. For instance, toxic materials such as dioxins, PCB and PCP may be present as contaminants in high concentration in wet soil, which, in addition, is interspersed with oily phases, which also contain toxic halogenated hydrocarbons. In such a case all processes known as chemically applicable to the dehalogenation of halogenated hydrocarbons will fail, because it is not possible to carry out reactions with sodium or potassium in wet soil and because dehalogenating reactions with less reactive metals, like magnesium, aluminum or iron, cannot be used under these conditions. Furthermore, it is impossible to reach the halogenated hydrocarbons in the described heterogeneous mix in order to manage any detoxifying, i.e. dehalogenating chemical reaction. All methods which have been tried so far in order to solve this sort of problem result in an exceptionally costly isolation of the halogenated hydrocarbons, either through extraction with solvents or through thermal stripping processes, for instance in a rotary kiln, and a subsequent high temperature incineration.
However, if the contaminated soil is largely dried thermally or chemically and subsequently disintegrated mechanically to form a pulverulent material, to which a reducing finely dispersed metal along with a hydrogen donating compound and, if necessary, an amine as a reaction accelerator is added and this homogeneous pulverulent mix, which can easily be handled, is heated up for a short time to an appropriate temperature it is possible to dehalogenate the halogenated hydrocarbons because all reaction partners necessary in a dehalogenating reaction are close together in a finely dispersed, i.e. in a highly reactive form. As a matter of fact, the dehalogenation reaction takes place though the reaction is, on closer examination, still a heterogeneous one and it is not easily to recognize, how the halogenated hydrocarbons, adsorptively bonded to the solid particles, can migrate onto the surface of the neighboring metal particles.
However, said course of reaction can be understood if one presumes that the adsorptively bonded halogenated hydrocarbons have access to the metal surface via the gas phase according to their vapor pressure under said reaction conditions. This agrees with the fact that the reaction time can significantly be decreased through the addition of hyperstoichiometric quantities of metal.
The way mentioned in the preceding example to produce a dry pulverulent matrix through a combination of thermal and mechanical processing steps yields good but not optimum results, especially under economic aspects. Best results can be achieved through reacting the oily and wet contaminated soil by means of a dispersing chemical reaction (DCR Technology, e.g. according to DE 23 28 778). For this purpose the contaminated soil is reacted with calcium oxide in a quantity, which is related to the water content present. Said quantity has to be stoichiometric if a complete removal of water is necessary, for instance for a dehalogenating reaction with sodium. The quantity may be significantly lower, for instance about 10 to 15% with respect to the weight of the soil, if metals like iron or aluminum are to be applied for the dehalogenation; in this case the excess moisture in the gas phase does not create any trouble. In a preferential form of implementation the calcium oxide has been rendered hydrophobic.
Calcium oxide can be homogeneously distributed in a wet and oily soil only on the condition that it is strongly hydrophobic. The hydrophobizing compounds should not contain any reducing functional groups in order to avoid a competitive metal consuming. It is of special advantage to render the calcium oxide hydrophobic through the addition of long-chain amines, e.g. stearylamine.
A thermally or chemically pre-dried solid formulation manufactured through grinding or through a dispersing chemical reaction (DCR) can be rendered completely dry through the addition of aluminum alcoholates. Since an alcohol is being formed through the hydrolysis of the alcoholate an additional application of a hydrogen donating compound is not necessary.
On heating a homogeneous pulverulent mix of a solid formulation produced by means of a dispersing chemical reaction (DCR) and a finely dispersed metal a ready and complete dehalogenation will occur.
Amines may be added to the extent of at least 0.1 equivalent per halogen in order to accelerate the dehalogenating reaction with reducing metals. However, this is not necessary if a reaction temperature is applied high enough with respect to the chemical properties of the metal. It seems that under these reaction conditions the thermally induced motions are able to prevent the metal surface from being sealed with reaction retarding layers or to remove such layers.
Amines applicable within the scope of the present invention are aliphatic primary, secondary and tertiary amines or diamines or amines with additional functional groups, in particular amino alcohols, or mixes of said amines or mixes of said amines with hydrogen donating compounds selected from other chemical families.
In case of the application of metals of particular reactivity, e.g. sodium, the residence time necessary in the heating zone will be around some seconds at 200xc2x0 C. and will increase to the same extent as the temperature decreases. In case an amine is added as an accelerator the residence time will be around 2 seconds at 80xc2x0 C.
With less reactive metals, such as iron, aluminum, zinc, the residence time is about 2-15 seconds at about 350xc2x0 C. and will be significantly higher for lower metal concentrations. In these cases the reaction mix will be held in a cyclic process as long as necessary to complete the dehalogenating reaction. An additional application of a hydrogen donating compound with hydrogen in a functional group, e.g. an alcohol, to generate the reduced hydrocarbon from the radical ion intermediate is not necessary, because at said high temperatures hydrogen will be split off even from a mere hydrocarbon compound.
Liquid halogenated hydrocarbons or halogenated hydrocarbons with a low melting point or solutions thereof in liquid inert solvents such as waste oil, transformer oil, hydraulic oil and the like can, in a first step, be dispersed by chemical reaction (DCR), homogenized with metals and further processing aids, i.e. amines and alcohols in a second step, and reacted in a third processing step. It is particularly effective and economic too if less reactive metals, such as aluminum and iron, are suspended in the halogenated hydrocarbons containing liquid, if necessary together with amines and alcohols, and then chemically dispersed by means of calcium oxide (DCR). Thus an extraordinary homogeneous solid formulation is formed in which each collection of agglomerated particles can be regarded as a small reactor in which all reaction partners are extremely intimately packed up together. This pulverulent mix, which can easily be blown, on heating for a short time up to 200 to 400xc2x0 C., for instance through a momentary contact with a hot metal surface of a reactor, e.g. a solid flow reactor, yields a complete and fast dehalogenation.
Liquid halogenated hydrocarbons or halogenated hydrocarbons with a low melting point or solutions thereof in liquid inert solvents such as waste oil, transformer oil, hydraulic oil and the like cannot be managed in the same way, i.e. in a synchronous dispersing reaction, if reactive metals, such as sodium, are involved. In this case the halogenated hydrocarbons containing liquid must be chemically dispersed separately, if necessary together with amine and alcohol, and the resulting pulverulent dispersion subsequently homogenized with the metal. In this context it is of particular advantage to finely disperse the metal beforehand on any finely dispersed inert carrier, for instance by means of a planetary ball mill, in order to achieve an extremely finely dispersed final mix of both the halogenated hydrocarbons containing dispersion and the finely dispersed reducing metal.
Metals of particular reactivity dispersed on a finely dispersed carrier own pyrophoric properties. For a better handling of such materials they have to be rendered inert through wrapping in inert materials or through mechanical or chemical hydrophobizing.
Halogenated hydrocarbons in sediments, oil sludge and in similar heterogeneous materials can be processed technically more simply if the contaminated material and the reaction partners are worked up separately and mixed finally for reaction. For instance, the heterogeneous halogenated hydrocarbons containing material as well as the mix of amine and alcohol can be transformed, individually, into pulverulent formulations by means of a dispersing chemical reaction (DCR) The resulting finely dispersed pulverulent solid formulations from each dispersing process can be mixed mechanically with the finely dispersed metal. Said final mix is then fed into the reactor for heating. This gives better results than a dispersing reaction directly applied to a mix of all components, i.e. to the heterogeneous contaminated materials along with amine, alcohol and metal.
The finely dispersed metal as such or on a carrier as well as the finely dispersed pulverulent formulations containing the amines and alcohols can be added to the pulverulent solid preparations containing the halogenated hydrocarbons all at once or in portions or continuously during the course of the dehalogenating reaction via a separate reactor inlet.
As regards the technical implementation of dehalogenating reactions the following variants are of practical importance.
Halogenated hydrocarbons present as contaminants in solids, which are free from water or almost free, e.g. ash and filter dust, can be dehalogenated continuously or discontinuously in a reactor tube, in an intensive or compulsory mixer, in a rotary kiln, in a fluidized bed reactor, in a packed solid bed reactor or in a solid flow reactor.
In a matrix free from water sodium can be added; in a matrix which is almost dry magnesium can be added. In an originally water containing matrix sodium can be used only in case the material is thermally or, more advantageous, chemically dried beforehand, for instance through the addition of absorbing agents such as calcium oxide and aluminum alcoholate.
Halogenated hydrocarbons as contaminants in soil or soil-like materials, such as sediments, industrial residues and by-products, which contain water, must be completely dried for a dehalogenation reaction with alkali metals and must be largely dried for a dehalogenation with other metals. For a dehalogenation in a solid flow reactor or in a fluidized bed reactor the matrix must be disintegrated to form a pulverulent material in order to achieve a optimum homogeneous distribution of all reaction partners in the solid matrix.
Drying can be carried out thermally or chemically through the addition of absorbing agents. An appropriate disintegration can be achieved by grinding the dried material. A solid dispersion with optimum properties can be manufactured through disintegration and drying the water containing matrix in one step by means of a dispersing chemical reaction (DCR) using hydrophobized calcium oxide.
The benefits of the present invention can be summarized as follows:
By means of the dehalogenation of halogenated hydrocarbons contained in a finely dispersed pulverulent solid formulation with reducing metals through a short-term heating on temperatures ranging from 80 to 400xc2x0 C. according to the present invention chemical dehalogenating reactions have been made applicable which otherwise would not allow a complete and economic dehalogenation of halogenated hydrocarbons.
Due to the numerous feasible variants regarding the execution of the dehalogenating reactions according to the invention it is possible now to solve environmental problems with halogenated hydrocarbons in most practice-relevant cases, in particular with halogenated hydrocarbon in waste oil, in oily residues and in polluted soils or soil-like materials even in critical compositions. The reaction temperature necessary to dehalogenate halogenated hydrocarbons within a solid matrix with reducing metals can significantly be decreased through the addition of amines.